Silt pollution of waterways from construction sites; a problem the industry must learn to address

ADAS' Kirk Hill examines the issue of silt pollution from construction and discusses some of the things that industry can do on-site to tackle the problem.

One of the biggest causes of water pollution from the construction and demolition industry in England and Wales is silt laden water run-off. This is a huge issue and represents the source of around 40 per cent of the industry's damaging water pollution incidents. With the Environment Agency still deciding how to fully address this problem, in light of the Water Framework Directive, the onus is on the construction industry to respond first.

Any pollution of a watercourse is already an offence under the Water Resources Act 1991. However, this does not just apply to industrial chemicals but to many other substances that some people may mistakenly feel are innocuous. Eroded soil is a perfect example of one such substance that many do not take seriously as they consider it a natural substance. But, if a significant amount of this 'silt' enters a watercourse, it can deoxygenate the water, block the gills of fish and smother aquatic plants and invertebrates, starving them of light and oxygen. Any ill effects on spawn or spawning fish can also leave a company vulnerable to prosecution under the Salmon and Freshwater Fisheries Act 1975.

When a construction site strips the topsoil and thus removes the vegetative cover, it takes away the land's strongest defence against erosion. The remaining surface has no shield or binding element to protect it from rainfall and run-off which will erode away the soil. When there is no plant matter to slow it down and the surface has become compacted by the use of heavy machinery, the rate of run-off increases and the scouring effect is exacerbated. If machinery continues to operate in wet conditions, it churns up the ground, releasing soil particles that become suspended in the surface water. If this water is subsequently allowed to flow into a watercourse, then there is a risk of environmental harm and prosecution.

With the unrolling of the Water Framework Directive (WFD), the political spotlight is very much on the water quality of our streams and rivers. There is speculation that the testing of water quality before, during and after construction projects will become far more vigorous in the future with the consequence that construction companies will find themselves increasingly at risk of prosecution. This can have a serious impact on a business, such as in the case of a contractor involved in the construction of the new Wembley stadium who was recently prosecuted for allowing silt to pollute a watercourse. The cost to them was a £10,000 fine plus over £3,000 costs.

Although pollution from schemes in built up areas may be more likely to be identified and reported by a passing member of the public, relying on the remoteness of a project in the countryside does not mean a contractor can escape notice. In 2003, traces of silt pollution were identified 20km away from the installation works for a cross-country pipeline in Scotland. This incident of pollution caused the death of over 50,000 fish in a downstream fish farm and the companies involved were fined a total of £30,000.

Silt pollution doesn't only bring financial problems; it also causes lost time, inconvenience and damaged reputation. In addition, there may be a third party client, such as a water company, whose own image will suffer by association.

The construction industry's historic approach to silt pollution has, all too frequently, been to do nothing until a problem arises. Some firms, however, do seek to address the issue. This may consist of a level of up-front staff training, such as highlighting the dangers of pumping silt-laden water from an excavation to a ditch or watercourse, and some businesses may also create settlement lagoons where there is space to do so. These measures are appropriate, but generally the industry is still not addressing the issue adequately at the planning stage.

So, what measures can the industry adopt? For a comparatively small cost, companies could commission a site assessment to identify those areas that will be particularly vulnerable to erosion during construction. The science for this already exists; it is just a case of applying it. The next stage is to consider properly appropriate site management techniques. These will entail staff training and, where necessary, planning for the 'end of pipe' solutions such as settlement lagoons, silt fences and de-silting machines. However, the strain on these systems would be reduced if the emphasis changed from 'how do we control silt-laden water' to actually limiting the creation of silt-laden water in the first place. The effect of this will be less treatment required and lowered vulnerability to pollution incidents, environmental damage and prosecution.

Site management should also include designs to cater for subsurface and surface water flows before they reach the main construction area. This may be in the form of pre-construction underdrainage, or the innovative positioning of topsoil mounds and surface 'grips' to direct water flows away from the areas of concern and prevent the build-up of surface water.

This needs to be designed properly, particularly on linear projects, such as roads and pipelines, however. There have been numerous cases in the past where water has been diverted away from the working area without any consideration for the original, natural and legal route of the water.

This can leave aquatic and semi-aquatic environments deficient of the water upon which they rely. It can also deplete a landowner's water resource while overloading that of another, causing flooding and crop loss. Poorly designed schemes can lead to litigation.

Linear projects also suffer from their 'channel like' nature. A linear working strip creates a funnel effect by concentrating flows along a single route, building up the volume of flow the longer the strip. This can create a river effect that scours away the soil on its route.

Time and time again, I have seen sites where the lack of a soils and drainage management approach has not only created a risk of pollution but also seriously held up the construction project because the ground conditions have become so poor that works can no longer physically proceed.

This can create huge downtime costs, which may be borne by the contractor or even passed on to the project promoter if 'act of god' is claimed. Even if the project is not totally stopped, drier ground conditions provided by good site management can mean works proceed quicker. Space may also be saved as it may be possible to rely less on settlement lagoons.

In cases of temporary land take, such as cross-country pipeline installations, good up-front soils and drainage planning and design can provide benefits from quicker, cheaper and better quality land reinstatement. This in turn can mean reduced disputes and compensation to landowners who will be able to return the land to its normal use earlier and more effectively than heavily damaged land.

By Kirk Hill.

For further information about managing construction run-off, contact Kirk Hill at ADAS on 01432 821026.